1. Technical Field
The present invention relates to a relay device which generates a database from data received from a plurality of communication devices which send and receive data, and relays data stored in the database to the respective communication devices. The present invention relates more particularly to a relay device capable of reducing the communication load on communication lines and also reducing the computational load on each communication device and the relay device itself, and to a communication system including the relay device, and a communication system.
2. Description of Related Art
In recent years, a system, which connects a plurality of communication devices and causes the communication devices to exchange data and perform various processes in cooperation with each other by assigning functions to the respective communication devices, has been used in a variety of fields. For example, in the field of vehicle-mounted LAN (Local Area Network) provided for vehicles, ECUs (Electronic Control Units) are used as communication devices, and each of the ECUs is arranged to perform a specialized process and exchange data with each other, thereby achieving a system with a variety of functions.
With the specialization of the functions of the respective communication devices and an increase in the number of functions capable of being performed by the respective communication devices, the number and types of communication devices connected to a communication medium also increase. Further, since a variety of functions are expected to be performed by the system, the communication devices need to share data and cooperate with each other, and an amount of data to be transferred is increased.
An increase in the amount of data sent and received through a communication line causes a delay or loss of data due to collisions. A considerable delay or loss of data is a fatal problem for driving assist functions, such as control of brake, performed by the ECUs.
Therefore, it is general to provide a plurality of separate communication lines and connect the ECUs to different communication lines, respectively. By grouping the ECUs which share data, it is possible to reduce wasteful use of the communication lines. Moreover, in order to efficiently use the communication lines for an increase in the types of ECUs, it may be possible to separate the ECUs according to the types of data to be sent and received and connect the ECUs to communication lines of different communication speeds. In these structures, different communication lines are connected with a relay device (gateway device) for controlling the sending and receiving of data.
Japanese Patent Application Laid-Open No. 2005-159568 discloses a technique in which when priority information for identifying the order of priority of data is added to data to be sent and received and a relay device sends and receives data between different communication lines, a priority level is determined from the priority information on the received data and data with a higher priority is sent first so that, even when the communication load on the communication lines is increased, sending of data with a higher priority is not delayed much.
Even when the communication devices are divided into a plurality of groups, if the relay device transfers all the data necessary for controlling the respective communication devices, the amount of data sent to the communication lines is not reduced. Therefore, Japanese Patent Application Laid-Open No. 2008-22158 discloses a technique in which data received from the communication devices are temporarily stored in a database, and data required by each communication device is appropriately retrieved from the database and sent.